Tree: The whole tree structure will break up if we drag-n-drop a parent node onto a child node.

Description

The whole tree hierarchy will break up if we drag-n-drop a parent node onto a child node. It is also present in the example given in the file : '/dojo-release-1.1.1/dijit/tests/tree/test_Tree_DnD.html'. In the Collections tree example given, just drag-n-drop the Fruits node onto the Citrus node. The whole tree structure will break-up producing exception. Ideally, it should not be allowed to drop a node onto its child node.

(In [20268]) Refs #7140
Fixes the simple case where a tree node is dragged into one of it's child nodes. This does not solve the more complex issue of a tree node with multiple parents being dragged into a child which cannot be readily determined by the tree. Such cases will have to be solved by the user, with whatever data store they happen to be using.